1. Field of the Invention
The invention pertains in general to method and apparatus for forming a magnetic image. More particularly, the invention provides for the generation of a magnetic image through the influence of optically induced dimensional variations upon the coercivity of a magnetostrictive material.
2. Description Relative to the Prior Art
The method and apparatus for forming a magnetic image in accordance with this invention is based on two phenomena: magnetostriction and piezoelectricity. Piezoelectricity is the property of certain dielectric crystals wherein a difference of electric potential is developed across them as a result of applied mechanical stress. Conversely, the application of a voltage between certain faces of the dielectric crystal produces a mechanical distortion of the material. This reciprocal relationship is referred to as the piezoelectric effect. The phenomenon of generation of a voltage under mechanical stress is referred to as the direct piezoelectric effect, and the mechanical strain produced in the crystal under electric stress is called the inverse piezoelectric effect.
Magnetostriction is descriptive of the property of certain materials which undergo a change in dimension when exposed to a magnetic field. These changes are extremely small in most substances, but they may be comparatively large in those which show ferromagnetic or similar behavior. Conversely, the existence of strain in a typical ferromagnetic material causes a change in its internal anisotropy energy that is, in turn, reflected primarily in a change in the coercivity of the material. Such strain can produce either an increase or decrease in coercivity depending on the characteristics of the material being used and the direction of the strain. In general, the magnetization of the material is affected indirectly insofar as either the coercivity is changed or demagnetizing fields influence the material.
For certain purposes, it is appropriate to rigidly fix a layer of piezoelectric material to a layer of magnetostrictive material such that any elastic strain produced in the magnetostrictive material is transmitted to the piezoelectric material which in turn generates an electric signal. Involving therefore a double energy conversion, such a device is useful as a sensor for detecting one form of energy and providing a reading in another form. For example, in U.S. Pat. No. 3,909,809, a magnetostrictive-piezoelectric device senses magnetic domains propagated in a sheet of magnetic bubble-domain supporting material. The magnetic energy associated with the bubble-domain is converted to elastic energy by means of the magnetostrictive material. The elastic energy is then in turn converted to electrical energy by means of the piezoelectric material. Finally an electrical output which represents the presence of a bubble domain is taken from across the piezoelectric material.
In addition to layering the magnetostrictive and piezoelectric materials, the interconversion of magnetic and electrical fields can also be achieved by the utilization of a composite material grown in situ, consisting of a piezomagnetic and piezoelectric component (see, for example, "Piezoelectric/Magnetic Material", Elect. Opt. Systems Design, Sept. 1974, page 5). For example, a molten eutectic mixture of barium titanate and cobalt ferrite may be allowed to solidify unidirectionally to obtain such a composite structure.